Brandon Moore wrote:
Thinking to take advantage of fortuitous heap layout of some Haskell values for interfacing with C, I've written the following function:
addressOf :: a -> Ptr () addressOf x = x `seq` unsafeCoerce# (Box x) data Box x = Box x
For example,
data A = A {-# UNPACK #-} !(Ptr Word8) {-# UNPACK #-} !CInt
main = let a = A nullPtr 12 p = addressOf a `plusPtr` 4 in do x <- peek p :: IO Int y <- peek p :: IO Int print (x, y) prints (0, 12)
One thing I don't understand is that this fails if I use Just rather than inventing my box type. I suppose the info table for Just is set up to support a vectored return for pattern matching on Maybe?
Yes, exactly right.
(the commentary isn't very clear here. The section http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution#Ret... says, in full: "Return Convention Direct Returns Vectored Returns" )
Sorry, I haven't got around to writing that section yet!
The reason I'm messing about with this stuff is that I'm pretty sure passing p to C code would give a usable pointer to struct a {char *; int;};
Obviously my plot will be spoiled if the GC comes along and relocates the value while C code is trying to use it.
Yes.
Are there any other pitfalls with this approach?
The GC pitfall not enough for you? :) Well, future GHC versions might change the representation under your feet, since we don't consider the heap object representation to be stable, user-visible stuff. Cheers, Simon